1. Field of the Invention:
The invention in general relates to steam turbine control systems, and particularly to a control arrangement wherein a load setpoint is modified in response to changes in power system frequency.
2. Description of the Prior Art:
In the operation of a steam turbine generator power plant, a steam source produces steam which is provided to a turbine system consisting of one or more turbines. The regulation of the steam through the turbine system is governed by the positioning of a plurality of steam admission valves and as the steam expands through the turbine, or turbines, work is extracted and utilized by an electrical generator for producing electricity which is supplied to a power system network through a circuit breaker arrangement.
When the circuit breakers are open, the torque, as produced by the inlet steam, is generally used to accelerate the turbine shaft from turning gear to synchronous speed. As long as the circuit breakers are open the turbine is spinning with no electrical load and it is operative in a speed control mode. Once the shaft frequency is synchronized to the frequency of the power system network, the circuit breakers are closed and power is delivered to the power system network by the generator.
With the circuit breakers closed, the net torque exerted on the turbine rotating assemblies controls the amount of power supplied to the power system network, while shaft speed is governed by the frequency of the power system network. Control of steam inlet under these conditions is generally referred to as load control and during which, the turbine speed is monitored and utilized in a speed feedback arrangement for purposes of regulating the power delivered, by control of the steam admission valves.
In the speed feedback arrangement during load control, the actual turbine speed is compared with the rated turbine speed to generate a speed error signal which is utilized in the control of the steam admission valves. In a typical operation, the power system frequency increases and decreases by small amounts due to constant changes in load, thereby resulting in a corresponding change in turbine speed. In such situation there is a possibility that the steam admission valves would constantly move up and down for small frequency deviations in an attempt to correct the situation. Accordingly, the speed feedback arrangement normally is provided with a deadband such that the speed has to deviate by some predetermined amount before correction is undertaken.
A problem arises in that in actual practice the system frequency, and accordingly the turbine speed, may vary about a value which deviates from the rated speed by an amount exceeding the deadband. For example, during the night there may be less of a demand for power, thus causing the system frequency (and turbine speed) to increase. Conversely, during the day there may be a greater demand for power, thereby causing the system frequency (and turbine speed) to decrease. The deviations may average out to the correct frequency, although, for minutes or even hours the system may be operating above or below rated values.
Such deviations therefore could be considered as normal for the operating conditions and yet the steam admission valves would constantly be moving up and down in response to small frequency deviations. The present invention allows operation at a speed value other than the rated speed and without the objectionable constant movement of the steam admission valves in response to small deviations about the speed value.